1. Field of the Invention
The invention relates to a high pressure discharge lamp of the short arc type. The invention relates especially to the side shape of the electrodes of a high pressure discharge lamp of the short arc type.
2. Description of the Prior Art
Recently a high pressure discharge lamp of the short arc type has been used for example as a light source in a photolithography process which is a production process for a liquid crystal color filter. The radiant light used here contains an intensive line spectrum at a wavelength of 365 nm or a wavelength of 436 nm.
On the other hand, there is a market demand for enlargement of the color filter and a shortening of the exposure duration. Furthermore, there is a demand for an increase in the amount of radiant light of the high pressure discharge lamp of the short arc type and especially an increase in the amount of radiant light at a wavelength in the vicinity of 365 nm is greatly desired.
Conventionally, it is known that the amount of radiant light of a high pressure discharge lamp of the short arc type is in a proportional relationship to the electrical input for a discharge lamp. This means that the amount of radiant light can also be increased when the electrical input for the discharge lamp is increased. To increase the electrical input for the discharge lamp there are the following methods:
1. increase the distance between the electrodes and thus the emission length of the high pressure discharge lamp of the short arc type
2. increase the amount of mercury to be added to the discharge lamp and thus operate the lamp in a state with a higher overpressure
3. increase the input current for the discharge lamp
The above described methods however have the following defects:
For Method I:
The emission part becomes larger than in the normally used point light source lamp due to the increase in the emission length. In the case of use as a light source in an exposure device for photolithography a point light source is desirable in conjunction with an irradiation optics system. The above described prolongation of the emission length is therefore not suited for a light source of this exposure device. It can no longer be used in practice, even if the amount of radiant light is improved.
For Method 2:
Since the internal pressure of the high pressure discharge lamp of the short arc type becomes great, there is a problem with respect to the mechanical strength of the emission tube. In a conventional high pressure discharge lamp of the short arc type there are many cases of a construction in which the vapor pressure of the contained mercury during operation is a pressure which approaches the upper limit of the internal pressure intensity of the lamp. In operation with a high pressure which is higher than the above described pressure, a high pressure discharge lamp of the short arc type is destroyed. This means that the method in which the amount of mercury added is increased more than in a conventional high pressure discharge lamp of the short arc type and in which the lamp is operated with a higher overpressure cannot be used to increase the amount of radiation.
In Method 3:
When the lamp current increases, the peak area of the anode is heated by the increase of the electron emission current; this leads to an increase in the temperature of the anode part. Of the heat generated in the anode there is normally heat which is emitted to the outside by heat conduction of the anode and heat which is emitted to the outside from the anode surface by radiation. In the method in which the lamp current is increased however the heat emitted to the outside is insufficient compared to heating by the increase of the electron emission current. As a result, thermal vaporization of the anode component is accelerated as a result of the temperature increase of the anode. This results in the disadvantages of blackening of the inside wall of the emission tube, shortening of the lamp service life, and similar disadvantages.
To eliminate these disadvantages a process was proposed in which the efficiency of thermal radiation from the anode is increased and in which the anode temperature is reduced.
For example, Japanese patent disclosure document SHO 39-11128 discloses that the anode side is provided with grooves with a V-shaped structure. Specifically it is described that there are cooling grooves with a depth of roughly 1 mm to 3 mm and an opening angle of 90xc2x0, that at the same time tantalum carbide is sintered onto the surfaces of these cooling grooves and that in this way the thermal irradiation from this anode surface is increased even more. In this process however there were the disadvantages that depending on the anode temperature carbon is released, that in this way blackening of the emission tube of the high pressure discharge lamp of the short arc type occurs or that carbon migrates to the electrode tip and that the electrode melts.
Furthermore, Japanese patent disclosure document HEI 9-231946 discloses that tungsten powder is sintered onto the anode side and that the heat emission capacity of the electrode surface is increased. FIG. 9 shows this arrangement. In a given surface area of an anode 90 fine-particle tungsten sinter layers 91 are formed. These fine tungsten particles have a grain size from roughly 0.1 microns to 100 microns. The area is enlarged by the measure that the anode surface is provided with them as sinter layers. This arrangement increases the amount of thermal radiation from the electrode surface. The attempt is made to lower the electrode temperature by this measure.
In this arrangement the thermal radiation from the electrode can be increased compared to the case in which a tungsten powder is not applied. When the electrical input for the discharge lamp is increased more, the cooling of this electrode however becomes insufficient. As a result the disadvantage is that the heat emission from the electrode is insufficient.
The object of the invention is to improve the thermal radiation characteristic of the electrodes in a high pressure discharge lamp of the short arc type in which the input power for the lamp has been increased to increase the amount of radiant light and to reduce the electrode temperature with high efficiency. Furthermore, the object of the invention is to be able to suppress or reduce vaporization of the electrode material from the tip area of the anode by reducing the electrode temperature with high efficiency, and to be able to reduce wear, thermal distortion and the like of the electrode tip and as a result to keep the emission of the discharge lamp stable over a long time.
The object is achieved as claimed in the invention in a high pressure discharge lamp of the short arc type, in the emission tube of which there is a pair of electrodes, in that at least part of the side of the above described electrode is provided with a groove area, that the depth D of this groove area is within 12% of the electrode diameter and that the relation D/P between the depth D of the groove area and the pitch P between the grooves is greater than or equal to 2.
The object is achieved as claimed in the invention in that the above described groove area consists of V-shaped grooves.
This object is moreover achieved as claimed in the invention in that the bottom area and/or the uppermost area of the above described groove area is/are provided with a curved surface.
The object is moreover achieved as claimed in the invention by providing the tip of the above described electrode with a conical part in which the above described groove area is formed.